The present invention generally relates to a method of placing carbon nanotubes (herein after “CNTs”) on a substrate. In particular, the invention relates to selective placement of charged CNTs on a pre-patterned surface having an oppositely charged, self-assembled monolayer.
CNTs can be semiconducting and therefore are of interest as channel materials for Field Effect Transistors (herein after “FET”). Accordingly, methods of placing CNTs on a substrate for use in FETs are being explored.
One approach to placing CNTs on a substrate involves directed assembly of CNTs from a suspension. In this approach, a substrate is patterned to define areas to which the CNT will have an affinity. The affinity is due to functionalization of either the substrate or the CNT to promote bonding between the substrate and the CNT.
In one instance, to place CNTs on a substrate, the prior art stamps a substrate with an organic compound to create a substrate having hydrophilic and hydrophobic regions. The hydrophilic region is the original substrate surface and the hydrophobic region is the area stamped with the organic compound. The substrate is immersed in a solution of CNTs and dried to leave CNTs on the hydrophilic regions. However, the CNTs on the surface of the substrate are bundled (i.e. a group of CNTs twisted together in a rope-like fashion) and/or multilayered. Bundled or multilayered CNTs are undesirable because a transistor made from them requires higher voltage to turn on an off. The described method has another drawback in that a solution of CNTs is not able to reach recessed hydrophilic areas having small widths (around or less than 200 nm). As a result, CNTs will be placed in large hydrophilic areas while small hydrophilic features remain uncovered. Accordingly, a CNT placement method based upon hydrogen bonding (a type of dipole bonding) can result in poor selectivity.
In other methods, the prior art places CNTs on a substrate by first functionalizing the CNT and then placing the CNT directly on the substrate. However, such methods result in a low density of CNTs on the surface.
Therefore, a need exists for a method of selectively placing a monolayer of high density CNTs on a substrate with minimal bundling.